Of course, we did find out what happened, eventually. By eventually, I mean by a vision from The Powers a few hours later; the longest, most painful vision of my life, but thankfully the last one anybody will ever receive, according to them. We learned about Illyria's visit to them…and about what she did. Why she did it…we can only speculate.
She always sought power…and she got it. For one, shining moment, she became more powerful than any being had ever been. Using the essence of The Wolf, The Ram and The Hart, she modified their ultimate end. The shockwave of their doomsday rolled across the world, through the sky, along the ground, and into the very core of the earth. Travelling at the speed of sound, it covered the globe. Those who saw it coming, human and demon alike, ran in terror at its approach. In it's path it left only those beings of good. Vampires, pure demons, and evil demons: all of them, dust.
Illyria selflessly sacrificed herself to wipe these beings from the face of the Earth, taking the Senior Partners with her. After all, what being has more power than that which can end every apocalypse in one fell swoop? What could immortalise you more than ending an age, and initiating a new one? What is grander than giving the gift of life to countless billions, most of them yet to be born?
But…she did more than that. She sealed this world from the other ones. All possible gateways were smashed. Never again will the Earth be menaced by demons from other dimensions. This place is now a shining citadel in a sea of darkness, safe for all eternity. How? The details are complicated, but the long and the short of it is she integrated the Key into the fabric of reality, sealing it, much like welding a steel door shut.
How did she discern evil from good? She didn't; the Partners did. They were keenly aware of which beings were evil, and she used this awareness to home in on them. The Partners were fuel for the fire, so to speak. What ultimate irony that their own master plan would be their final undoing.
Why didn't she just tell us, instead of fighting? Her plan would only work before the bomb got too much energy, lest even she be unable to control it, and it was near the critical point. Besides, the powers pointed out, would we have believed her when she said she was going to save the world, even though everything we thought she wanted was right in front of her? No. She knew that, and she acted accordingly.
How did she destroy an entire dimension? She used the energy of the doomsday device to destroy the higher plane where The Partners resided, forcing them to assume mortal form. They were taken totally by surprise, all their pillars of support stripped, leaving them with only their very finite, body-bound powers.
As for the war…well, it hasn't stopped, but it is dying down. With the forces of darkness forever silenced, the flames have stopped being fanned. A lot needs to be done to rebuild. Lives have been lost, cities and nations have been destroyed, but mankind is at the dawn of a new, golden age. The only foe left to deal with is his own capacity for evil, but that is a different type of battle.
The Shanshu is a strange one. It had to be fulfilled, but how? It specified the vampire with a soul would become human. Neither Spike nor I is strictly human, but neither are we vampires any more. We have all the strengths and none of the weakness now. We can enjoy food, but we still like blood. Apparently what happened is that both Spike and I had equal points on the champion scale, and the Shanshu "tried" to fulfil us both. This wasn't possible, so it naturally readjusted itself via some bizarre magical law I don't really understand. It treated the term "Vampire" as plural, and "restored" us as well as it could with its divided energy. It's all very complicated, but the bottom line is, we're humans with benefits.
What about Buffy, Spike and I? Well…
Our job is done.